My 50 watt #183 with footswitchable FET build.

[Matt J]

Installed the new NTE red LED and it lights up just fine with the 330r dropping resistor. It's in the "Goldilocks Zone" - not too bright, not too dim, just right!

As I mentioned above, I find high-brightness LED's far too much at rated current, either for the front panel or for the footswitch. In this case, you have maybe 4-5mA average running through it. I like to power the panel LED from the relay supply, which keeps a small current flowing through the regulator and eliminates the extra junk needed to power it from the filament string. Next time I think I'll integrate the regulator with the relay supply board and clean that area up a bit more.

Great point!

I'm going to have to remember that for my future builds!
- Matt J.

[Matt J]

Update:

i set up a jig (really a mess of wires) and completed the biasing sets for the FET. No popping capacitors this time!! I used a perf board with small pots installed to adjust the drain and source resistors to the correct voltage. From there I measured each pot's resistance and used a resistor that can closed to each measurement. Someday, after I get some practice etching my own circuit boards, I would like to build a board with pots and test resistors install to bias FET for future builds.
8.2K/150k divider = 17V
NTE452 Drain: 9.9V
NTE452 Source: 2.32V

That's as close as I feel I need to get, so I'm happy to leave things as-is from now and install the FET board in the chassis.

I've finished tidying things up with the power section, and have started attaching the leads to the preamp board.

I'm going on a road trip tomorrow to the mountains, so I probably won't have a chance to get back into things until Monday evening or Tuesday as I inch ever closer to going through the first full power up.

- Matt J.

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[Matt J]

Gutshots of how the chassis currently is.

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[Matt J]

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[martin manning]

I used a perf board with small pots installed to adjust the drain and source resistors to the correct voltage. From there I measured each pot's resistance and used a resistor that can closed to each measurement. Someday, after I get some practice etching my own circuit boards, I would like to build a board with pots and test resistors install to bias FET for future builds.
8.2K/150k divider = 17V
NTE452 Drain: 9.9V
NTE452 Source: 2.32V

Assuming you are following the bias procedure I posted, you should only be adjusting the source resistor value. The drain resistor stays as-is, and you won't know where you are with the 8k2 until you power it from the B+5 node (the divider is not in the circuit at this point). A simple breadboard set up could be used to find the source resistor value in a few minutes using just the 10k, the FET, and a 10k pot.

[Matt J]

I used a perf board with small pots installed to adjust the drain and source resistors to the correct voltage. From there I measured each pot's resistance and used a resistor that can closed to each measurement. Someday, after I get some practice etching my own circuit boards, I would like to build a board with pots and test resistors install to bias FET for future builds.
8.2K/150k divider = 17V
NTE452 Drain: 9.9V
NTE452 Source: 2.32V

Assuming you are following the bias procedure I posted, you should only be adjusting the source resistor value. The drain resistor stays as-is, and you won't know where you are with the 8k2 until you power it from the B+5 node (the divider is not in the circuit at this point). A simple breadboard set up could be used to find the source resistor value in a few minutes using just the 10k, the FET, and a 10k pot.

Good point! I forgot that the divider is not in the circuit using this bias method! With the 8.2k resistor in place, I ended up keep Rd as 10k, Rs came out as 2.7k, though I ended up using a 2.2k I already had on hand for that spot. I'll wait and see how things work voltage wise (and how things sound) when I fire it up with the B+.

- Matt J.

[Matt J]

The mountain trip got cancelled due to bad weather, so I've been able to spend most of the morning working on the amp.

The preamp board has been installed with the tubes-side leads trimmed and soldered in. I've been going over it with the continuity function on my DMM and haven't found any accidental shorts to ground. Up next I will connect the leads to the control pots/switches and finish up the PI. Then it is on to heater wiring and final continuity checks before bringing out the current limiter for an initial fire up.

I've used wire with a heavier coat and 600v rating for the B+ feeds and the PI plates. It's a little awkward to maneuver around the other PI leads, but I wanted something rated above the normal 300v NTE hook up wire I've mostly used. The schematic and layout show the voltages in this section creeping over the 300v mark, so I prefer to be safe rather than sorry when using that wire here.

I went ahead and set the wiper OD entrance trimmer to 22k above ground per the original layout. I will adjust that to my tastes later when everything is working ok.

Since I'm running this as a 50 watt, I've connected the NFB wire to the 8 ohm tap off of the OT secondaries.

Oh, and in case anyone is curious, the white shielded wire is RG-188 I got from Pasternack.

The tubes I will be initially using are three JJ gold pin 12AX7s with low noise and matched triodes for the preamp, and a set of matched EHX EL34s in the power section.

- Matt J.

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[norburybrook]

Looks great. A lot tidier than my amps


All I'll say is when you do fire it up and everything is working, be aware that adding a boosted FET to the OD can put it over the top. It's fine on my second gen, but on my recently built BM, if the gain is past midway on the FET then in OD it will squeal when engaged as there's too much gain.


The BM does have more gain then the other ODS amps so perhaps you'll be OK.



M

[Matt J]

Looks great. A lot tidier than my amps


All I'll say is when you do fire it up and everything is working, be aware that adding a boosted FET to the OD can put it over the top. It's fine on my second gen, but on my recently built BM, if the gain is past midway on the FET then in OD it will squeal when engaged as there's too much gain.


The BM does have more gain then the other ODS amps so perhaps you'll be OK.



M

That's exactly what I'm aiming for, M!

If I wanted something with wild amounts of gain, I would have tried to build a BM or just a JCM800. With this, I want the FET to just add a slight amount hair to the clean tone. In fact, if I knew of a way to keep the FET from engaging while the OD is on I would wire it that way!

It's not the easiest to explain, but the kind of FET tone I have in mind for would be similar to Albert King's live tone from his Fillmore East show.
(Of course having a Korina Flying V and playing left handed strung backwards would help! )
https://www.youtube.com/watch?v=OrLJkFH369M

- Matt J.

[Matt J]

Update:
Finished up most of the preamp board, still some tidying up left to do.

From here, construction-wise all that is left to do is wire up the OD relay and the heaters, then it will be time to get out the current limiter and give things a smoke-test.

Concerning the OD relay, from Martin's diagram I wanted to make sure that I wire it properly the first time. Is it correct that there's a jumper from NC1 to NO2 that connects to the clean output .05uf coupling cap? I've drawn on your diagram in MS Paint to show what I am asking about.

- Matt J.

OD Relay board.jpg

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[martin manning]

Concerning the OD relay, from Martin's diagram I wanted to make sure that I wire it properly the first time. Is it correct that there's a jumper from NC1 to NO2 that connects to the clean output .05uf coupling cap?

Correct.

[norburybrook]

Looks great. A lot tidier than my amps


All I'll say is when you do fire it up and everything is working, be aware that adding a boosted FET to the OD can put it over the top. It's fine on my second gen, but on my recently built BM, if the gain is past midway on the FET then in OD it will squeal when engaged as there's too much gain.


The BM does have more gain then the other ODS amps so perhaps you'll be OK.



M

That's exactly what I'm aiming for, M!

If I wanted something with wild amounts of gain, I would have tried to build a BM or just a JCM800. With this, I want the FET to just add a slight amount hair to the clean tone. In fact, if I knew of a way to keep the FET from engaging while the OD is on I would wire it that way!

It's not the easiest to explain, but the kind of FET tone I have in mind for would be similar to Albert King's live tone from his Fillmore East show.
(Of course having a Korina Flying V and playing left handed strung backwards would help! )
https://www.youtube.com/watch?v=OrLJkFH369M

- Matt J.

Matt,

Yes I Know exactly what you mean and the FET does that great. I had the same thought about making it impossible to have the FET and OD on together but couldn't get my non technical mind to come up with a solution....other than a warning sticker on the FET footswitch!!!

Looking great though, you should have a wonderful amp.

M

[dorrisant]

You could wire in another relay as an interlock... The interlock relay would be activated when the OD is engaged. Run the ground for the FET relay through the normally closed set of contacts of the interlock. Any time you would engage the OD it would kick the FET off... Just a thought.

[martin manning]

There’s a much easier way: Just run the ground path for the FET switch through the unused side of the (SPDT) OD switch in the footswitch. When OD is on, FET boost is inactive.

[dorrisant]

There’s a much easier way: Just run the ground path for the FET switch through the unused side of the (SPDT) OD switch in the footswitch. When OD is on, FET boost is inactive.

That will definitely work. What I proposed will work even if the footswitch is not connected. I usually install front panel switches so that you can still access all functions without the FS.